Simulation study on the growth variation of fish with special reference to the mortality by cannibalism.

In respect to the higher initial mortality in fish by cannibalism, computer simulation of individual growth variation was made in terms of coefficient of variation (CV) and skewness (sk) of frequency distribution. Three normal distributions of initial body length (l0), specific growth rate in exponential growth or growth coefficient in Bertalanffy's growth (lambda) and asymptotic value of the same growth (linf.) consisted of each 8,000 in number were supposed. In all the case, changes of the body length (maximum, minimum and mean), CV and sk in the cannibalistic group were compared with those of the control groups. At t0 larger individual preys on smaller one between the randomly sampled two. From t1 the body length ratios which enable the predator to prey on the victim were gradually expanded in the course of time in different way according to the growth equation. After preying, the predator grows additionally by lambda/40 regardless of growth equations. Thus, survival rates were maintained on the level of 6-12% at t7 in all the cases. In exponential growth CV and sk of both cannibalistic and control group continuously increased in N-N correspondent type and N-N type of the combination of l0 with lambda, but slight increase and decrease of sk was observed in N-N complex type of the cannibalistic group. In any type of the combination the values of CV and sk of the cannibalistic groups were always smaller than those of the control groups. In Bertalanffy's growth, CV of the control groups gradually converged on CV of the given asymptotic value after initial increase in all the cases. CV of the cannibalistic groups changed similarly to the control groups, but converged on the smaller values than the asymptotic CV of the control groups. Sk of the cannibalistic groups finally converged on the positive values after irregular changes in negative and positive values, contrary to the marked increase in negative values from the beginning and final convergence on 0 in the control groups. In pseudo-asymptotic growth both CV and sk of the control groups increased asymptotically in all the cases. CV of the cannibalistic groups converged on the smaller values than the asymptotic values of the control groups after initial increase. Sk of the cannibalistic groups finally converged on the similar values to those of the control groups after irregular changes in positive values except N-N correspondent type.